Electromagnetic transducers of variable reluctance type are used in many applications for stimulation through bone conduction such as in hearing aids, transducers for hearing diagnostic purposes and in communication systems.
In the manufacturing of all types of electromagnetic transducers of variable reluctance type it is of utmost importance that air gaps are small and stable. These should be small in order to maintain a maximum efficiency level and stable so as not to change over time or with differences in temperature/moisture or external mechanical influence. Air gaps are needed between one or more magnetically conductive components in the magnetic circuit of the seismic mass side (the reaction side) and one or more magnetically conductive components in the magnetic circuit of the transducer's load side (actuation side). By using a spring suspension arrangement between the seismic mass side and the load side these can be kept apart across the distance of the air gap-s. Manufacturing transducers with small and mechanically stable air gaps sets higher demands on the tolerances of components used and often requires that a fully assembled transducer is dismantled for readjustment of the air gaps. This can be achieved by, for example, grinding the surface facing the air gaps. Readjustment to the air gaps is the reason why manufacturing costs of these transducers are relatively high compared with transducers of the moving coil type.
Electromagnetic transducers of variable reluctance type have been improved through various inventions such as the U.S. Pat. No. 6,751,334, which describes a transducer according to a new principle, the BEST™ technique and through another SE-C-522,164 invention which describes how iron loss (eddy current loss) can be reduced by lamination. One specific property of these transducers is that they have so called “balanced air gaps”.